1. Field of Invention
The present invention relates to insufflation of recycle dust in a kiln. More particularly, the present invention relates to novel apparatus and processes for the injection of an oxidant into a fluidized recycle dust stream to improve calcination of recycle dust in a rotary kiln used for the calcination of minerals such as cement, lime, dolomite, magnesia, titanium dioxide, and other calcined materials.
2. Brief Description of Related Art
Cement may be manufactured by mixing and reacting raw materials such as calcium carbonate, silica, alumina, iron oxide, magnesium carbonate, etc. in a high temperature rotary kiln. A composition including the above material first undergoes a drying and heating process. Next, the material undergoes a calcination process, in which carbonate minerals are converted into mineral oxides. The above minerals are then recombined at much higher temperatures to produce a product comprising calcium silicates and calcium aluminates. The resulting product, referred to as clinker, is then cooled, pulverized, and mixed with additional ingredients to form cement.
FIG. 1 is a schematic, cross-sectional illustration of an exemplary rotary kiln. Referring to FIG. 1, rotary kiln 100 includes an inclined, rotatable clinker bed 110 having an inlet 112 for receiving raw clinker material 114 and an outlet 116 for releasing clinker material 114. Burner 118 provides a flame that extends into the interior of kiln 100 to define a combustion zone necessary to increase the temperature of the raw clinker material 114 that moves through kiln 100, and to enable the various chemical reactions that transform the raw material into clinker 114. It will be noted that in some modern cement plants, a significant amount of heat energy may be provided to the raw material prior to its arrival in kiln 100. In operation, clinker raw material 114 is fed into inlet 112 and flows along rotatable clinker bed 110, where it is subjected to heat from burner 118.
Depending on the raw product quality, kilns can be designed for wet, semi-wet, semi-dry and dry processes. The specific process determines the kiln dimensions. Each of these processes commonly uses an inclined rotary kiln. Raw materials are fed to the kiln at the elevated end and flow in a direction opposite the flow of combustion products originating from the main burner. The combustion of fuel and oxidant in burner 118 provides the required heat for the efficient processing of the material.
In many cases, particularly in long dry kilns, the calcining process results in a substantial amount of dust entrained in flue gases 120. The dust entrained in the flue gas system comprises completely and partially processed product, unburned carbon from fuel, various condensates and used refractory wall lining from the kiln. The dust, collected by the bag-house or cyclone separators, can be as much as 20% of the total raw material fed to the kiln. Under the government land reclamation laws and the Resource Conservation and Recovery Act (RCRA), the cement dust is considered a hazardous substance and the land disposal costs can be significant. Accordingly, it is both environmentally and economically desirable to recycle as much of this dust as possible.
FIG. 2 is a schematic illustration of a conventional dust recycling system 200. Kiln 204 emits flue gas dust from flue 208. Flue gas dust is collected from the bag-house 210 and/or cyclone type separator(s) 212. The flue gas dust is stored in a dust collection vessel 216, also referred to as a storage bin. In most cases, dense-phase conveying of flue gas dust is performed in a repetitive batch operation. The recycled flue gas dust enters an air-lock vessel commonly referred to as a transmitter 220 at atmospheric pressure. At a desired time (e.g., when the transmitter is full) the transmitter's inlet valve is closed and air compressor 224 provides a compressed air supply to increase the air pressure in transmitter 220, typically to a pressure between 80 and 100 pounds per square inch gauge (psig) (5.5 to 6.9 Bar). The air flow through transmitter 220 fluidizes recycled dust, which then flows under pressure in recycle dust pipe 228 back to rotary kiln 204. At a desired time (e.g., when transmitter 220 is empty), the air pressure at transmitter 220 may be lowered to atmospheric pressure and the cycle may be repeated. It will be appreciated that multiple transmitters could be used to provide a continuous operation.
The dense-phase conveying system depicted in FIG. 2 can achieve a high throughput over a long distance using a recycle dust pipe 228 having a relatively small diameter (e.g., 8 inches (0.2 m) to 10 inches (0.25 m)). A conventional measuring device 218 may be used to measure the mass of recycled dust conveyed to the kiln. A conventional controlled air management system 226 with pressure switches may be used to pressurize/depressurize each transmitter 220. Line boosters 232 may be used to provide additional air pressure at regular intervals along recycle pipe 228, if necessary, to reduce plugging of dense-phase recycle dust pipe 228. It will be appreciated that the amount of air required for transporting recycled dust depends upon parameters including the average dust particle size, the diameter and length of dust recycle pipe 228, and the desired flow rate. It will also be appreciated that the recycle dust pipe can be installed within the burner pipe.
Previous dust recycling efforts include a technique known as insufflation. Insufflation recycles flue gas dust using a dust injection pipe to feed recycle dust to the kiln's main burner. Conventional insufflation systems have the capability to recycle only a relatively small proportion of the total dust generated by the kiln, primarily because the recycle dust inhibits the main burner flame, thereby reducing the efficiency of the kiln. Among the undesirable effects of dust laden flame are a longer flame, high CO emissions, increased fuel consumption, incomplete clinker formation and lower yield.
Referring again to FIG. 1, kiln 100 includes a recycle dust pipe 140 disposed adjacent burner 118 for feeding recycle dust to burner 118. Dust pipe 140 is commonly disposed above burner 118 such that recycle dust exiting dust pipe 140 flows under the force of gravity into the flame of burner 118. Techniques exist to increase the amount of dust that can be recycled by a kiln. One technique is to provide an oxygen lance 130 underneath the main burner as described in U.S. Pat. No. 5,007,823 and U.S. Pat. No. 5,572,938. Oxygen lance 130 increases the amount of oxygen available to the burner. In addition, oxygen may be added through the existing air-fuel burner using an oxygen pipe 132 as shown in U.S. Pat. No. 5,572,938. In each of these configurations, oxygen is provided to the main flame to increase the main flame reaction rate.
Each of these insufflation techniques suffers from some drawbacks. First, their efficiency is limited. U.S. Pat. No. 5,007,823 claims that the insufflation techniques disclosed therein achieve, at most, a 65-75% increase in the amount of dust that can be recycled, when compared to a kiln in which no oxygen is added. Second, the use of a separate dust injection pipe leads to localized flame quenching at the dust injection location due to the heating of the dust by the main flame. This causes the flame to become relatively colder and less stable, particularly at high dust injection rates. Third, the oxygen injection rate and the dust recycle rate should be balanced to maintain a desired kiln temperature profile and product quality. Increasing the oxygen injection rate may cause localized overheating of the product and the furnace refractory. Conversely, increasing the recycle dust injection rate may cause localized quenching or cooling of the flame, flame instability, longer flame, higher CO emissions, increase in the cold end kiln temperature and incomplete clinker formation. Maintaining a desired relationship between the dust injection rate and the oxygen injection rate to effectively balance these effects is difficult.
Accordingly, there is a need in the art for improved systems and methods for recycling flue gas dust from kilns.